Conventional power amplifier boards are traditionally built as heavy or thick metal backed printed circuit boards (PCBs) called a brick or pallet board. The metal consists of a variety of alloys, most commonly used is copper and aluminum. The metal is formed as a metal block which is attached to a back side of the PCB. Typically, the PCB is mechanically mounted to the metal block with screws, solder, or adhesive bonding, although alternative methods, such as sweat soldering, can also used. This is a labor intensive process.
The width and length of the metal block match that of the PCB. The metal is used to dissipate heat and provide electrical grounding. A PCB typically includes one or more heat generating components, such as driver components, amplifier transistors, power circuits, circulators, and the like. Certain electronic components generate a large amount of heat. The heat generated by such a component is pulled away from the component and dissipated across the metal block. The metal block is in turn attached to a secondary heat sink to further dissipate the heat. Copper is the most commonly used material for the metal block due to its high thermal conductivity. Aluminum can be used to reduce weight and allow better handling, however, the thermal conductivity is not as good as copper.
Critical to efficient heat transfer from the heat generating components to the metal block is proper contact between the PCB and the metal block. However, contact of the PCB back side plane to the metal block can be compromised. For example, heat may cause warping of either the PCB or the metal block, the contact surfaces may oxidize over time, the PCB is only mechanically tied to the metal block through the mounting screws, there is no localized connection to help with isolated surface heat, and only the heat generating device of the PCB is physically contacting the metal block.
Additionally, the above assembly requires manual placement of the PCB onto the metal block after components have been mounted onto the PCB and mostly requires special tooling. Further, the above assembly includes extra assembly processes and handling that could jeopardize the finished product. Examples of such extra assembly processes and handling include mounting that can be jeopardized by pre-assembly processes, oxidation or surface contamination that can effect the efficiencies of the copper backing, tolerance controls for copper backing that can impact contact with the device mitigating the thermal performance, milling of the backing that increases the cost of final assembly, and isolated contact made through the mounting screws.
The metal block of this type is often excessive in size and is not suitable for mass production. Other techniques are used to more selectively position smaller sized metal blocks within the PCB substrate, such metal blocks commonly referred to as metal coins or pallets. Embedding such metal coins is a post-lamination process. First the PCB is formed, then a coin cavity is formed in the back side of the laminated PCB. A thermally conductive adhesive sheet, bonding film, or solder is applied within the coin cavity and the metal coin is positioned within the coin cavity. The soldering process requires a process with tight controls, incurs extra cost in tooling, and special tooling is required. The use of solder may result in long term reliability issues related to entrapped flux or solder voiding. The soldering process has to be executed at elevated temperatures and deformation of the PCB or metal coin can happen at these higher temperatures. Adhesives are not as problematic as solder, but adhesives are costly, can react poorly to additional elevated thermal cycles seen in assembly, and can lead to long term reliability issues if the bond is compromised.
The assembly is typically mounted to a heat sink for heat transfer from the embedded coin. Critical to efficient heat transfer is a planar surface on the back side of the PCB. The depth of the coin cavity and the thickness of the adhesive or solder must be finely regulated so that a bottom surface of the metal coin is co-planar with the back side surface of the PCB. This is difficult to consistently achieve. A common result is an uneven disposition of the coin bottom surface and the back side surface of the PCB.
FIGS. 1-4 illustrate an exemplary fabrication process for embedding a coin in a PCB according to conventional process and resulting structure. First a laminated stack is formed. FIG. 1 illustrates a cut out side view of an exemplary laminated stack. The laminated stack includes one or more top layers 4, one or more bottom layers 8 and intervening conductive layer 6. The laminated stack also includes outer conductive layer 2 and outer conductive layer 10. The top layers 4 and the bottom layers 8 can each represent a single non-conductive layer, such as a dielectric, or a multi-layer stack made of one or more conductive layers and one or more non-conductive layers. The conductive layers are each electrically conductive layers made of a metal, such as copper, that is patterned into electrically conductive traces, or interconnects. The outer conductive layers are often plated, such as with copper plating, and finish plated with an inert metal, such as gold, to prevent oxidation. The outer conductive layers and finish plating are etched after the lamination step. Vias can be formed to interconnect one or more of the conductive layers. The number of layers may vary depending on the application.
Once the laminated stack is formed, cutouts are made to receive the heat generating device and the metal coin. FIG. 2 illustrates a cut out side view of the laminated stack from FIG. 1 with cutouts. Device cutout 16 is formed in the front side of the laminated stack, through layers 2, 4, 6, and 12. Device cutout 16 is sized to receive a heat generating component. Coin cutout 18 is formed in the back side of the laminated stack, through 8, 10, and 14. Coin cutout 18 is sized to receive a metal coin.
The metal coin is secured in the coin cavity 18 by an adhesive. FIG. 3 illustrates a cut out side view of the laminated stack of FIG. 2 with an adhesive 20 added within the coin cutout 18. The adhesive 20 is positioned on the exposed surface of the conductive layer 6. A metal coin 22 is then inserted in the coin cutout 18, as shown in FIG. 4. A top surface 26 of the metal coin 22 contacts the adhesive 20. The adhesive 20 can be a thermally conductive adhesive but can also be a non-conductive type, and in addition to securing the metal coin 22 in position within the coin cutout 18, the adhesive 20 functions as a thermal interface material between the metal coin 22 and the conductive layer 6.
The assembly shown in FIG. 4 is typically attached to an external heat sink. To maximize heat transfer from the assembly to the heat sink, a planar bottom surface of the assembly is desired. However, due to the imprecise nature of forming the coin cutout 18 and applying a thickness of the adhesive 20, a bottom surface 24 of the metal coin 22 is often not co-planar with a bottom surface 28 of the laminated stack, as shown in FIG. 4. In general, the metal coin does not fit perfectly within the cavity. This results in both lateral gaps between the metal coin 22 and bottom layers 8. This also results in the metal coin either extending outward from the PCB backside surface or recessed within the coin cutout. In either case, a thermal interface material (TIM) is applied to compensate for the backside surface irregularities resulting in areas having greater TIM thickness than others. Unfortunately, TIM is not as thermally conductive as the material of the metal coin and outer bottom conductive layer, such as copper, and the areas of greater TIM thickness due to non-planar back side surfaces results in decreased thermal transfer efficiency.